Ernst added during an interview on CBS's "Face the Nation" that she thinks Trump is "very frustrated" with immigration.

"I think that she has done a good job. I think the president is just very frustrated, of course, with what we see at the border. We do have a large number of immigrants that are trying to get over the border illegally and we do need to make sure that we've got plenty of surveillance, that we have plenty of officers ready to respond," Ernst said.

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"I think her response has been well so far, we as Congress need to step forward and do the next steps," she continued.

Ernst added, however, that it is "entirely up to" Trump whether Nielsen should continue serving in her role.

"I think that she's done a good job, but of course, any of our secretaries or Cabinet members serve at the pleasure of the president," she said. "It's entirely up to him who serves in those positions."

Earlier Sunday, Trump said on "Fox News Sunday" that Nielsen needs to be "much tougher" on the border. His comments came amid speculation that she could soon depart from his administration. He also said Sunday that he was considering changes to as many as five Cabinet members.

"I like her very much I respect her very much I’d like her to be much together on the border. Much tougher. Period," Trump said of Nielsen.